Current Concepts in the Management of Tumors of the Skull Base

Akio Morita, MD, Laligam N. Sekhar, MD, and Donald C. Wright, MD

Approaches to the management of cranial base tumors, including surgery, radiotherapy, chemotherapy, hormonal therapy, and combinations of therapy, are individually tailored according to anatomy, tumor classification, and biologic aggressiveness.

Background:  Due to their involvement with critical neurovascular structures, tumors located in the cranial base present challenges to neurosurgeons and are associated with high morbidity and mortality.
Methods:  Rates of tumor control, complications, patient outcomes, and recurrences were extracted and summarized from two decades of our surgical and radiological treatment follow-up and review of the medical literature.
Results:  Recent advances in surgical techniques involving cranial base approaches have made surgical intervention safer and curative resection more likely.  In managing benign tumors, surgical resection is the gold standard for treatment.  While immediate complications are still significant, long-term outcomes in most cases are excellent.  Focused radiosurgery using a gamma knife or linear accelerator has produced favorable outcomes, and it improves the management of small or minimally symptomatic cranial base tumors.  For slow-growing malignant tumors, extensive surgery followed by radiotherapy achieves the best outcome.  In managing highly malignant tumors, outcome is determined by the effects of chemotherapy and radiotherapy.  On some occasions, surgery is needed to obtain greater control of highly malignant tumors.
Conclusions:  Skull base tumors are relatively common, and management of these tumors is rapidly evolving.  The combination of surgical excision using cranial base techniques, radiosurgery, fractionated radiotherapy, and chemotherapy should be individually tailored based on the location and pathological aggressiveness of the tumor and the symptomatology of the patient.

Introduction

    Management of tumors in the cranial base has been challenging for neurosurgeons and other medical specialists for more than a century.¹ Since tumors are located deep in the skull and can involve important neurovascular structures, surgical treatment was associated with high morbidity and mortality. Until recently, radiation therapy was the standard palliative measure. However, during the 1970s and 1980s, neurosurgeons and head and neck surgeons^2-4 introduced advances in surgical instrumentation and developed new approaches to the cranial base. Simultaneously, focused stereotactic radiotherapy was introduced to treat localized tumors in a more refined manner.⁵ With this combination of modern techniques, recent management of skull base tumors has dramatically improved.⁶

General Concepts

Classification and Management

    Various types of tumors involve the skull base. As depicted in Table 1, the most frequent types of tumors treated in our institutions are meningiomas, pituitary adenomas, and schwannomas. However, the numbers can be significantly influenced on a referral basis. In general, we can classify and discuss the management of the cranial base tumors according to their individual histopathologic types. Since there are numerous types of tumors, we have limited their classifications to three groups based on biologic aggressiveness, and in nine territories based on their location. This classification system not only simplifies our choice of treatment modalities, but also clarifies our understanding of the pathophysiology in clinical presentation. Biologic aggressiveness directs our decision on which mode of treatment is the best from an oncological standpoint, and the location of tumor and clinical presentation provide information on the risks that are involved in treatment.

Biologic Aggressiveness

    We group skull base tumors in roughly three pathologic entities: benign, slow-growing (low-grade), and fast-growing (high-grade) malignancies. Table 2 shows examples of each category.

    Benign tumors grow in an expansive fashion and induce clinical symptoms by exerting pressure on certain neurovascular structures. Hence, the reduction of the mass effect and, if possible, the complete excision of tumors are the gold standard in treating such lesions. Usually, symptoms caused by the mass effect can resolve fairly quickly. However, there are always associated surgical risks that may depend significantly on the location and extent of the tumor. With the recent development of diagnostic techniques, asymptomatic or minimally symptomatic tumors are being discovered in the skull base. With benign tumors, treatment modalities should be chosen based on observation, the excision of the tumor by surgery, or the control of tumor growth by stereotactic or conventional radiotherapy. The long-term benefits and risks of each procedure and the natural history of various tumor should be carefully assessed.^7,8

    Slow-growing malignancies such as chondrosarcomas, chordomas, low-grade esthesioneuroblastomas, and adenoid cystic carcinomas are best treated with a combined mode of surgical debulking and radiotherapy.^9-11 Sensitivity to radiotherapy is variable with each tumor type. Surgical resection should be as complete as possible, especially with radioresistant tumors such as chordomas. Cranial base approaches are useful to facilitate such procedures because they provide the surgeon with an excellent view from which to remove tumors and involved bones. Radiotherapy can involve either fractionated or focused radiosurgery depending on the size and extent of the tumor. Focused heavy particle irradiation is reported with a favorable outcome in cases with chordomas and chondrosarcomas.¹²

    Highly malignant tumors can be removed en bloc if the process does not involve critical structures. However, in most situations, piecemeal resection or sacrifice of a significant structure may be involved in surgical resection. Hence, radiotherapy with or without chemotherapy is the main treatment modality when the cranial base approach to a tumor may cause the loss of sensitive cranial structures.

Tumor Location

    Tumor location significantly affects our surgical strategy. Table 3 summarizes the classification of location, the frequently encountered tumors in each site, and the involved neurovascular structures and symptoms. The tumor’s relation to adjacent structures such as the optic apparatus should be considered. Tumors involved in more than one territory can present more complex problems. Because of the complexity in anatomical structures, the cavernous sinus (parasellar) and the petroclival areas are the most frequently discussed territory for the management of the skull base tumors.

Diagnostic Studies and Interventional Radiological Treatment

    Diagnostic studies for skull base tumors include magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), computed tomography (CT) scans, bone window and three-dimensional (3-D) CT scans, and angiography. MRI is currently an essential test to show the nature and extent of the tumor. Tumor appearance on T1- and T2-weighted images a